Frequency interpolating device and frequency interpolating method

ABSTRACT

The spectrum of a PCM signal is divided into bands. Combinations of a reference band inclusive of a highest frequency band and another band, one of the reference band and other band being normalized, are checked to identify a combination having a highest spectrum distribution correlation. The spectrum having the same distribution as the spectrum distribution of the reference band contained in the identified combination is scaled along an envelope function and added to a higher frequency side than the reference band to generate an output signal. A presence/absence of high frequency components of a PCM signal is detected. Only if there are high frequency components, the spectrum components are added to generate an output signal. It is therefore possible to recover a signal approximate to the original signal from either an original signal with the spectrum components in some bands being suppressed or a signal representative of an original signal containing no spectrum components in the bands or from a signal combining these two signals.

TECHNICAL FIELD

The present invention relates to a frequency interpolating device and afrequency interpolating method capable of improving spectrumdistribution of a band-limited signal.

BACKGROUND ART

Distribution of data of the MPEG1 audio layer 3 (MP3) format anddistribution of music and the like by frequency modulation (FM)broadcast, television sound multiplex broadcast and other methods areprevailing nowadays. These methods generally eliminate frequencycomponents of about 15 kHz or higher of music and the like in order toavoid an increase in data amount and an expansion of an occupied bandwidth to be caused by an excessively broad band.

Music and the like whose frequency components at a predeterminedfrequency or higher are removed have generally a poor sound quality.Signals substituting the removed frequency components are added toimprove the sound quality, as disclosed in JP Laid-Open Gazette No.7-93900.

According to the approach disclosed in JP Laid-Open Gazette No. 7-93900,a PCM digital audio signal is passed through a low pass filter and itsoutput signal is multiplied by a signal containing absolute valuecomponents of the output signal to generate distortion.

An audio signal reproducing apparatus disclosed in JP Laid-Open GazetteNo. 7-93900 generates harmonic waves by distorting the waveform of anoutput audio signal with a limitter circuit and the like. It isindefinite that such harmonic waves are approximate to those containedin the original audio signal.

The invention has been made to solve the above-described problemassociated with prior art. A first object of the present invention is toprovide a frequency interpolating device and a frequency interpolatingmethod capable of recovering a signal approximate to an original signalfrom a predetermined conversion signal obtained from a band-limitedsignal of the original signal, particularly a frequency interpolatingdevice and a frequency interpolating method capable of recovering anaudio signal of a high sound quality.

According to the prior art, even if the frequency components at apredetermined frequency or higher are not necessary to be removed, anaudio signal of music or the like is compressed into the MP3 format orthe like so that the band of the audio signal is generally limited.

Even if original sounds or the like represented by a PCM digital audiosignal have no frequency components higher than the pass band width of alow pass filter, a conventional device adds unnecessary high frequencycomponents not contained in the original sounds or the like. The qualityof an output audio signal is degraded more than it is passed through alow pass filter and additional signal processing is not performed.

Under such circumstances, a second object of the invention is to providea frequency interpolating device and a frequency interpolating methodcapable of recovering a signal approximate to an original signal evenfrom a signal mixed with a signal representative of the original signalwhose spectrum components in some bands were removed and a signalrepresentative of the original signal which has no spectrum componentsin these bands.

DISCLOSURE OF THE INVENTION

In order to achieve the first object of the invention, in a frequencyinterpolating device for receiving an input signal of an original signalwhose frequency components in a particular frequency band wassuppressed, approximately recovering suppressed frequency components andreproducing a signal approximate to the original signal, short periodspectra are obtained from a frequency band having frequency componentsnot suppressed, a short period spectrum in the suppressed frequency bandare estimated by paying attention to repetition of a spectrum pattern ata predetermined frequency interval, and in accordance with thisestimation, a signal containing the frequency components in thesuppressed frequency band is synthesized and added to the input signal.More specifically, in the frequency interpolating device of theinvention, the repetition of the spectrum pattern is judged from acorrelation coefficient between a spectrum pattern in a first frequencyband having a predetermined band width and frequency components notsuppressed near the suppressed frequency band and a spectrum pattern ina second frequency band adjacent to the first frequency band having thepredetermined band width.

If the spectrum pattern in the first frequency band and the spectrumpattern in the second frequency band have a high correlationcoefficient, a replica of the spectrum pattern having a correlation iscoupled to interpolate the frequency components in the suppressedfrequency band.

With this frequency interpolating device, a portion of a spectrum of asignal to be interpolated having a high spectrum distributioncorrelation is added along an envelope line to the high frequency sideof the signal to be interpolated to thereby expand the band. The addedspectrum can be regarded as some harmonic components of the originalspectrum. Therefore, if the signal to be interpolated has a limitedband, the signal with the expanded band is approximate to the originalsignal before the band limitation. If the signal to be interpolated isan audio signal, the audio signal of a high sound quality can berecovered from the signal with the expanded band.

In the frequency interpolating device of the invention, the intensitiesof the frequency components to be synthesized are determined from aspectrum envelope of the suppressed frequency band estimated from aspectrum envelope of the frequency band not suppressed. Preferably, theparticular frequency band is a high frequency band, and an upper limitfrequency of the first or second frequency band is a lower limitfrequency of a suppressed high frequency band.

If the interpolation band contains the highest frequency spectrum of thesignal to be interpolated, there is a high possibility that theinterpolation band itself is some harmonic components of the originalspectrum. The signal with the expanded band is more approximate to theoriginal signal before band limitation.

According to another aspect achieving the first object of the invention,the frequency interpolating device comprises: spectrum generating meansfor generating short period spectra of the input signal; spectrumpattern deriving means for deriving short period spectrum patternshaving a correlation in adjacent frequency bands having a same bandwidth; spectrum envelope deriving means for deriving spectrum envelopeinformation in the band whose frequency components are not suppressed;means responsive to the spectrum pattern deriving means and the spectrumenvelope driving means for synthesizing a frequency spectrum signal forinterpolating the suppressed frequency band; and means for adding thesynthesized spectrum signal to the input signal. In this device, thesynthesized spectrum signal contains the frequency components in thesuppressed frequency band, the derived spectrum pattern and the leveldetermined by the spectrum envelope information. Typically, the inputsignal is a PCM signal obtained by sampling and quantizing an analogaudio signal.

According to another aspect of the invention, there is provided afrequency interpolating method of receiving an input signal of anoriginal signal whose frequency components in a particular frequencyband was suppressed, approximately recovering suppressed frequencycomponents and reproducing a signal approximate to the original signal,wherein: short period spectra are obtained from a frequency band havingfrequency components not suppressed; a short period spectrum of thefrequency components in the suppressed frequency band is estimated inaccordance with repetition of a spectrum pattern in the frequency bandhaving frequency components not suppressed, and the estimated shortperiod spectrum pattern is synthesized and added to the input signal.

In order to achieve the second object of the invention, in the frequencyinterpolating device of the invention and in a frequency interpolatingsystem for receiving an input signal of an original signal whosefrequency components in a particular frequency band was suppressed,approximately recovering suppressed frequency components and reproducinga signal approximate to the original signal, the device and systemcomprise: means for judging whether the particular frequency band of theoriginal signal contains frequency components having a predeterminedlevel or higher and generating identification data representative of apresence/absence of the frequency components having the predeterminedlevel or higher; signal conversion means for suppressing the frequencycomponents of the original signal in the particular frequency band andsubjecting the original signal to a predetermined signal conversionprocess; means for superposing the identification data upon theconverted signal and transmitting the identification data and theconverted data; judging means for receiving a transmitted signal,checking the identification data contained in the signal, and judging apresence/absence of the frequency components in the particular frequencyband; branch control means for controlling to output the received signalto an external if said judging means judges that the particularfrequency band does not contain the frequency components and to inputthe received signal to succeeding signal processing means if the judgingmeans judges that the particular frequency band contains the frequencycomponents; and signal processing means responsive to the receivedsignal from the control means for performing an inverse conversionprocess of the predetermined signal conversion process and aninterpolation process of approximately synthesizing and adding thefrequency components in the suppressed frequency band. Morespecifically, the predetermined signal conversion process is a datacompression process and the inverse conversion process to be executed bysaid signal processing means is a data decompression process. Theinterpolation process to be executed by said signal processing meansincludes (i) a short period spectrum analysis process, (ii) a process ofderiving short period spectrum patterns in adjacent frequency bandshaving a correlation, and (iii) a process of deriving spectrum envelopeinformation.

With the frequency interpolating system, the identification data isgenerated which is representative of whether the spectrum of theoriginal signal is distributed to the suppressed frequency band. If theidentification data indicates an existence of a spectrum in thesuppressed frequency band, a portion of the spectrum with a highcorrelation of the signal to be interpolated is added along an envelopeto the high frequency side of the signal to be interpolated to therebyexpand the band. The added spectrum can be regarded as some harmoniccomponents of the original spectrum. Therefore, if the signal to beinterpolated has a limited band, the signal with the expanded band isapproximate to the original signal before the band limitation. If theidentification data indicates no existence of the spectrum in theinterpolation band, the signal to be interpolated is output withoutspectrum addition.

As a result, even if the received signal is a signal to be interpolatedhaving the suppressed spectrum components in some bands of an originalsignal, or a signal representative of the original signal which does notcontain the spectrum components in the bands, a signal approximate tothe original signal can be recovered. If the signal is an audio signal,the audio signal of a high sound quality can be recovered.

The above-described frequency interpolating system has an integratedarrangement of a signal transmission side (including an encoder) and asignal receiving side (including a decoder). The invention may beembodied only by the reception side (decoder side). In this case, afrequency interpolating device for receiving an input signal of anoriginal signal whose frequency components in a particular frequencyband was suppressed, approximately recovering suppressed frequencycomponents and reproducing a signal approximate to the original signal,comprises: means for receiving a first signal obtained by subjecting theoriginal signal whose signal components in the particular frequency bandwere suppressed to a predetermined signal conversion process and asecond signal superposed upon the first signal of identification datarepresentative of whether the particular frequency band of the originalsignal contains the frequency components having a predetermined level orhigher; judging means for checking the identification data contained inthe received signal and judging a presence/absence of the frequencycomponents in the particular frequency band; branch control means forcontrolling to output the received signal to each part if the judgingmeans judges that the particular frequency band does not contain thefrequency components and to input the received signal to succeedingsignal processing means if the judging means judges that the particularfrequency band contains the frequency components; and signal processingmeans responsive to the received signal from the branch control meansfor performing an inverse conversion process of the predetermined signalconversion process and an interpolation process of approximatelysynthesizing and adding the frequency components in the suppressedfrequency band.

Similar to the frequency interpolating device, in order to achieve thefirst object of the invention, there is provided a frequencyinterpolating method which comprises: a step of judging whether theparticular frequency band of the original signal contains frequencycomponents having a predetermined level or higher and generatingidentification data representative of a presence/absence of thefrequency components having the predetermined level or higher; a step ofsuppressing the frequency components of the original signal in theparticular frequency band and subjecting the original signal to apredetermined signal conversion process; a step of superposing theidentification data upon the converted signal and transmitting theidentification data and the converted data; a judging step of receivinga transmitted signal, checking the identification data contained in thesignal, and judging a presence/absence of the frequency components inthe particular frequency band; a branch control step of controlling tooutput the received signal to an external if the judging step judgesthat the particular frequency band does not contain the frequencycomponents and to input the received signal to a succeeding signalprocessing step only if the judging step judges that the particularfrequency band contains the frequency components; and a signalprocessing step, responsive to the received signal from the branchcontrol step, of performing an inverse conversion process of thepredetermined signal conversion process and an interpolation process ofapproximately synthesizing and adding the frequency components in thesuppressed frequency band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the structure of a frequency interpolatingdevice according to a first embodiment of the invention.

FIG. 2 is a diagram showing the structure of an analyzer.

FIG. 3(a) is a diagram showing the spectrum of an original audio signaland FIG. 3(b) is a diagram showing the spectrum of the audio signalwhose frequency components higher than a predetermined frequency wereremoved.

FIGS. 4(a) and 4(b) are diagrams showing examples of spectrumdistributions after interpolation.

FIG. 5 is a diagram showing the structure of a synthesizer.

FIG. 6 is a diagram showing the structure of a frequency interpolatingdevice according to a second embodiment of the invention.

FIG. 7 is a diagram showing the structure of a frequency interpolatingsection shown in FIG. 6.

FIG. 8 is a diagram showing the structure of a frequency interpolatingdevice according to a third embodiment of the invention.

EMBODIMENTS OF THE INVENTION

Embodiments of the invention will be described in detail with referenceto the accompanying drawings.

(First Embodiment)

FIG. 1 is a diagram showing the structure of a frequency interpolatingdevice according to a first embodiment of the invention.

As shown, this frequency interpolating device is constituted of ananalyzer 1, a frequency interpolating section 2, an interpolation bandadding section 3, and a synthesizer 4.

As shown in FIG. 2, the analyzer 1 is constituted of n delay units 11-0to 11-(n−1), (n+1) samplers 12-0 to 12-n and a filter bank 13 (where nis an integer of any of 1 or larger).

Each of the delay units 11-0 and 11-(n−1) outputs an input signal bydelaying it by one sampling period. A signal output from a delay unit11-k is supplied to a sampler 12-k (where k is an integer of any of 0 to(n−1)). A delay unit 11-j is supplied with a signal output from a delayunit 11-(j+1) (where j is an integer of any of 0 to (n−2)). The delayunit 11-(n−1) is supplied with a pulse code modulation (PCM) signalwhich is to be subjected to frequency interpolation by the frequencyinterpolating device.

Therefore, the delay unit 11-k outputs a PCM signal supplied from thedelay unit 11-(n−1) by delaying it by (n−k) sampling periods of the PCMsignal.

A PCM signal is a signal obtained by sampling and quantizing, i.e.,so-called PCM modulating, an analog audio signal such as a voice signal.The spectrum distribution of an audio signal represented by a PCM signalshows that the frequency components of an original audio signal shown inFIG. 3(a) higher than a predetermined frequency (14 kHz in the exampleshown in FIG. 3(b)) are removed.

Each of the samplers 12-0 to 12-n samples a supplied signal at asampling frequency of 1/(n+1)-th the sampling frequency of the PCMsignal to be subjected to frequency interpolation, and supplies thesampled signal to the filter bank 13.

As described earlier, the sampler 12-k is supplied with an output of thedelay unit 11-k. The sampler 12-n is supplied with the PCM signal to besubjected to frequency interpolation by the frequency interpolatingdevice, substantially at the same time when the PCM signal is applied tothe delay unit 11-(n−1).

The filter bank 13 is constituted of a digital signal processor (DSP), acentral processing unit (CPU) and the like.

As described earlier, the filter bank 13 is supplied with outputs of thesamplers 12-1 to 12-n.

The filter bank 13 generates first to (n+1)-th (n+1) signalsrepresentative of short span spectrum distributions of the inputsignals, by using a poly-phase filter, discrete cosine transform (DCT),lapped orthogonal transform (LOT), modulated lapped transform (MLT), aquadrature mirror filter (QMF), extended lapped transform (ELT) or thelike. Namely, the filter bank 13 converts a time sequential signal intoa frequency spectrum signal. The generated (n+1) signals are supplied tothe frequency interpolating section 2 and interpolation band addingsection 3.

It is assumed that the p-th signal generated by the filter band 13 is asignal representative of a spectrum distribution in the p-th lowestfrequency band among the bands obtained by dividing by (n+1) the shortspan spectrum distributions output from the samplers 12-0 to 12-n (wherep is an integer of any of 1 to (n+1)).

The frequency interpolating section 2 is constituted of a DSP, a CPU andthe like. Upon reception of the (n+1) signals representative of thespectrum distributions of the (n+1) bands from the filter bank 13, thefrequency interpolating unit 2 performs, for example, the followingprocesses (1) to (5) to determine a reference band to be used as aninterpolation band.

(1) In order to determine the interpolation band, the frequencyinterpolating section 2 first identifies a band (reference band) formedby coupling consecutive q (q is an integer in the range from 1 or largerto n or smaller) higher frequency bands among the bands represented bythe signals supplied from the filter bank 13. A mean square value X ofthe spectrum components of the reference band is calculated. The bandhigher than the highest frequency of the reference band is defined as aband which does not substantially contain the spectrum of an audiosignal represented by the PCM signal supplied to the analyzer 1.

(2) The frequency interpolating section 2 identifies a band (comparisonband) formed by coupling consecutive q (q is an integer in the rangefrom 1 or larger to n or smaller) higher frequency bands excluding theband having the highest frequency among the bands represented by thesignals supplied from the filter bank 13. A mean square value Y of thespectrum components of the comparison band is calculated.

(3) By using the mean square values of the spectrum components of thereference band and comparison band, the values of the spectrumcomponents of the comparison band are normalized. For example, a ratioY/X of the mean square value of the spectrum components of thecomparison band to the mean square value of the reference band iscalculated, and this ratio is multiplied by respective spectrumcomponents of the comparison band. A set of obtained products representsa spectrum distribution of the normalized comparison band.

(4) A correlation coefficient between the spectrum distributions of thereference band and normalized comparison band is calculated by a leastsquare method or the like.

In this case, the correlation coefficient is calculated by the frequencyinterpolating section 2 on the assumption that the frequency of eachspectrum of the comparison band is an original frequency added with adifference between the lowest frequencies of the reference band andcomparison band.

(5) The frequency interpolating section 2 calculates the correlationcoefficients by executing the processes (1) to (4) for all availablevalues of q and for all available combinations of the reference band andcomparison band. Of these combinations, the combination having a highestcorrelation coefficient is identified. Identification information of thereference band contained in the identified combination is supplied tothe interpolation band adding section 3.

The interpolation band adding section 3 is constituted of a DSP, a CPUand the like. Upon reception of the (n+1) signals representative of thespectrum distributions of the (n+1) bands from the filter bank 13, theinterpolation band adding section 3 identifies a function representativeof an envelope of the spectrum distribution of each band. By performingregression calculations or the like by using the identified function, anestimate value of a mean square value of the spectrum components to becontained essentially in the interpolation band which is a band on thehigher frequency band side than the highest frequency band (althoughthis interpolation band was suppressed by a band limiting process).

A single interpolation band or a plurality of interpolation bands may beused. The width of each interpolation band is assumed to be equal to thewidth of the reference band identified by the information supplied fromthe frequency interpolating section 2. If there are a plurality ofinterpolation bands, these bands are continuous without any overlap andthe interpolation band adding section 3 calculates the estimate value ofa mean square value of the spectrum components of each interpolationband.

Upon reception of the information for identifying the reference bandfrom the frequency interpolating section 2, the interpolation bandadding section 3 obtains the spectrum distribution of the interpolationband through scaling of the identified reference band.

Namely, the interpolation band adding section 3 first calculates themean square value of the spectrum components of the identified referenceband. Then, a ratio of the estimate value of the spectrum components ofthe interpolation band to the calculated mean square value of thespectrum components of the reference band is calculated. This ratio ismultiplied by each of the spectrum components of the reference band. Aset of calculated products represents a spectrum distribution of thereference band after scaling.

The interpolation band adding section 3 generates a signalrepresentative of the spectrum distribution of the interpolation band byconsidering the spectrum distribution of the reference band afterscaling as the spectrum distribution of the interpolation band. Thegenerated signal as wall as the signals supplied from the filter bank 13is supplied to the synthesizer 4.

Namely, the interpolation band adding section 3 supplies the synthesizer4 with a spectrum distribution (spectrum distribution afterinterpolation) obtained by adding spectrum components of theinterpolation band to the spectrum of the original PCM signal.

If the interpolation band adding section 3 considers the spectrumdistribution of the reference band after scaling as the spectrumdistribution of the r-th interpolation band as counted from the lowerfrequency side, it is assumed that the frequency of each spectrum of thereference band after scaling is an original frequency added with ahighest frequency of the reference band and a value (r−1) times thewidth of the interpolation band.

FIGS. 4(a) and 4(b) show examples of the spectrum distribution afterinterpolation.

In the example shown in FIG. 4(a), of seven bands (first to seventhbands) of an audio signal represented by an original PCM signal, acombination of the seventh and sixth bands has a highest correlationcoefficient. Namely, spectrum patterns have a repetition of one bandperiod. In this case, as shown in FIG. 4(a), a spectrum havingsubstantially the same distribution as that of the seventh band which isthe reference band is added to four interpolation bands A1 to A4.

In the example shown in FIG. 4(b), of seven bands of an audio signalrepresented by an original PCM signal, a combination of the sixth andseventh bands and the fourth and fifth bands has a highest correlationcoefficient. Namely, spectrum patterns have a repetition of a two-bandperiod. In this case, as shown in FIG. 4(b), a spectrum havingsubstantially the same distribution as that of the reference band (aband constituted of the sixth and seventh bands) is added to twointerpolation bands B1 and B2.

As shown in FIG. 5, the synthesizer 4 is constituted of a filter bank41, (n+1) samplers 42-0 to 42-n, n delay units 43-0 to 43-(n−1) and nadders 44-0 to 44-(n−1).

The filter bank 41 is constituted of a DSP, a CPU and the like. Asdescribed earlier, the filter bank 41 is supplied with a signalrepresentative of the spectrum distribution after interpolation outputfrom the interpolation band adding section 3.

The filter bank 41 generates (n+1) signals representative of the valuesof signals having the spectrum distribution of a supplied signal andsampled at (n+1) points, by using poly-phase filters, DCT, LOT, MLT, ELTor the like (e.g., converts a spectrum signal in the frequency domaininto a signal in the time domain). Of these generated (n+1) signals, ap-th signal (p is an integer of any of 1 to (n+1)) is supplied to asampler 42-(p−1).

It is assumed that the sampling period for the values of the signalsgenerated by the filter bank 41 is substantially equal to the samplingperiod of the samplers 12-1 to 12-n of the analyzer 1.

The p-th signal generated by the filer bank 41 represents the value atthe p-th earliest sampling time among the values obtained by sampling atthe (n+1) points and at an equal pitch the signal having the spectrumdistribution representative of the signal supplied to the filter bank41.

Each of the samplers 42-1 to 42-n converts a supplied signal into asignal having a frequency (n+1) times that of the supplied signal, andoutputs a PCM signal representative of the conversion result.

As described earlier, the sampler 42-(p−1) is supplied with the p-thsignal output from the filter bank 41. A sampler 42-(s−1) supplies itsoutput signal to an adder 44-(p−1) (where s is an integer of any of 1 ton). The sampler 42-n supplies its output signal to the delay unit43-(n−1).

Each of the delay units 43-0 to 43-(n−1) delays a supplied signal by oneperiod and outputs it.

A delay unit 43-k supplies its output signal to an adder 44-k (where kis an integer in the range from 0 or larger to (n−1) or smaller). Adelay unit 43-j is supplied with a signal output from an adder 44-(j+1)(where j is an integer in the range from 0 or larger to (n−2) orsmaller). As described earlier, the delay unit 43-(n−1) is supplied witha signal output from the sampler 42-n.

Each of the adders 44-0 to 44-(n−1) outputs a signal representative of asum of supplied two signals.

An adder 44-k is supplied with signals output from a sampler 42-k anddelay unit 43-k. An adder 44-m supplies its output signal to a delayunit 43-(m−1) (where m is an integer in the range from 1 or larger to(n−1) or smaller). A signal output from the adder 44-0 is an outputsignal of the frequency interpolating device.

An output signal from the adder 44-0 is a PCM signal having the spectrumdistribution after interpolation and obtained by sequentially outputtingthe signals output from the samplers 42-0, 42-1, . . . , 42-(n−1) and42-n at substantially the same period as that of the PCM signal suppliedto the analyzer 1.

Of the spectrum distribution after interpolation, the spectrumdistribution of the interpolation band added by the interpolation bandadding section 3 has a spectrum distribution corresponding to thespectrum distribution of the reference band contained in the combinationof the reference band and comparison band having the highest spectrumdistribution correlation. Therefore, the spectrum distribution of theinterpolation band can be considered as harmonic components of thereference band or comparison band. An output signal from the adder 44-0is therefore a PCM signal obtained through PCM of an audio signalsimulated to the audio signal before the band limitation. By reproducingthe audio signal from the output signal of the adder 44-0, the audiosignal having high sound quality can be recovered.

The structure of the frequency interpolating device is not limited onlyto that described above.

For example, the functions of the delay units 11-0 to 11-(n−1) and 43-0to 43-(n−1), samplers 12-0 to 12-n and 42-0 to 42-n and adders 44-0 to44-(n−1) may be realized by DSP and CPU.

The frequency interpolating section 2 may determine the interpolationband by calculating a numerical value representative of a correlationbetween the reference band and comparison band instead of thecorrelation coefficient, in accordance with the spectrum distribution ofthe reference band and comparison band.

The frequency interpolating section 2 may identify a combination betweenthe reference band and comparison band and thereafter supply theinformation for identifying the comparison band in the identifiedcombination to the interpolation band adding section 3. In this case,the interpolation band adding section 3 obtains the spectrumdistribution of the interpolation band through scaling of the identifiedcomparison band.

The frequency interpolating section 2 may normalize the comparison bandin the above-described process (3).

However, if the spectrum distribution of the interpolation band isobtained from the spectrum distribution of the reference band, there isa high possibility that the reference band itself is harmonic componentsof the comparison band because the highest frequency of the referenceband contains the highest frequency of the spectrum of the original PCMsignal. Therefore, if the spectrum distribution of the interpolationband is obtained from the spectrum distribution of the reference band, asignal output from the adder 44-0 becomes an audio signal more simulatedto the audio signal before the band limitation than if the spectrumdistribution of the interpolation band is obtained from the spectrumdistribution of the comparison band.

A signal to be interpolated by the frequency interpolating device isneither limited only to a PCM signal nor it is required to be amodulated signal of an audio signal.

Although the embodiment of the invention has been described above, thefrequency interpolating device of the invention may be realized by usinga general computer system without using a dedicated system.

For example, a program realizing the functions of the analyzer 1,frequency interpolating section 2, interpolation band adding section 3and synthesizer 4 may be read from a storage medium (such as CD-ROM, MOand floppy disc) and installed to realize the functions of the frequencyinterpolating device which executes the above-described processes.

The program may be presented on a bulletin board system (BBS) on acommunication line to distribute it. A carrier may be modulated bysignals representative of the program to transmit the obtained modulatedwave. An apparatus received this modulated wave demodulates it torecover the program.

The above-described processes can be executed by running and executingthe program under the control of an OS like other application programs.

If OS shares a portion of the processes or constitutes a portion of eachconstituent element of the invention, the program removing such aportion may be stored in a storage medium. Also in this case, thestorage medium stores the program for executing each function or step ofthe computer.

The previously-described first object of the invention can beeffectively achieved by the frequency interpolating device (or method)according to the first embodiment of the invention.

(Second Embodiment)

FIG. 6 is a diagram showing the structure of a frequency interpolatingdevice according to a second embodiment of the invention which canachieve the second object of the invention.

As shown in FIG. 6, the frequency interpolating device is constituted ofa high frequency component detecting section 1, a voice compressionsection 2, a voice decompression section 3 and a frequency interpolatingsection 4.

As shown in FIG. 6, the high frequency component detecting section 1 isconstituted of a high pass filter (HPF) 11 and a detector unit 12.

HPF 11 receives a PCM signal to be compressed, cuts the frequencycomponents at a predetermined frequency or lower, and supplies the othercomponents (high frequency components) to the detector unit 12. The PCMsignal to be compressed is also supplied to the voice compressionsection 2.

The PCM signal to be compressed is generated from an audio signalrepresenting a voice or the like as a change in voltage or current. Thecut-off frequency of HPF 11 is set higher than the upper limit frequencyof a band occupied by compression data of the PCM signal compressed bythe voice compression section 2. For example, if the upper limitfrequency of the band occupied by the compression data is about 14 kHz,the cut-off frequency is set to, e.g., about 16 kHz.

Upon reception of the high frequency components of the PCM signal fromHPF 11, the detector unit 12 detects the high frequency components andgenerates a detection signal. This detection signal is supplied to thevoice compression section 2 at the timing synchronous with the timingwhen the PCM signal to be compressed is supplied to the voicecompression section 2.

The voice compression section 2 is constituted of, for example, a DSP, aCPU, a multiplexer and the like. The voice compression section 2 hasalso a storage medium drive for reading/writing data from/to a storagemedium (e.g., CD-R).

Upon reception of the PCM signal to be compressed, the voice compressionsection 2 performs data compression by MP3, advanced audio coding (AAC)or another method. The upper limit frequency of a band occupied by dataobtained by data compression (compression data described above) is apredetermined frequency or lower.

The voice compression section 2 generates external data indicatingwhether the PCM signal contains high frequency components, in accordancewith whether the detection signal is supplied from the detector unit 12.

Specifically, upon reception of the detection signal from the detectorunit 12, the voice compression section 2 generates synchronously withthe detection signal the external data indicating that the PCM signalcontains high frequency components. On the other hand, if the detectionsignal is not supplied synchronously with the supply of the PCM signalto be compressed, then the voice compression section 2 generates theexternal data indicating that the PCM signal does not contain highfrequency components.

For example, the upper limit frequency of the band occupied by thecompression data is about 14 kHz and the spectrum distribution of thePCM signal supplied to HPF 11 (and voice compression section 2) is suchas shown in FIG. 3(b) (substantially no spectrum components at 14 kHz orhigher), then the detector unit 12 generates the external dataindicating that the PCM signal does not contain high frequencycomponents.

The voice compression section 2 records the compression data of the PCMsignal and the corresponding external data representative of whether thePCM signal contains high frequency components in an external storagemedium set in the storage medium drive.

The voice compression section 2 may have a communication controlapparatus constituted of a modem, a terminal adaptor or the likeconnected to an external communication line, instead of or incombination with the storage medium drive. In this case, the voicecompression section 2 transfers, via the communication line to anexternal, the compression data of the PCM signal and the external datarepresentative of a presence/absence of high frequency components of thePCM signal.

If the voice compression section 2 compresses the PCM signal in the MP3format, the external data is included in unshellery code.

The voice decompression section 3 has, for example, a DSP, a CPU and thelike as well as a storage medium drive. The voice decompression section3 reads the compression data of the PCM signal compressed by MP3, AAC oranother method and the corresponding external data from the externalstorage medium set in the storage medium drive. The read compressiondata is decompressed by MP3, AAC or another method to generate a PCMsignal representative of decompression data. This PCM signal and thecorresponding external data read from the storage medium are supplied tothe frequency interpolating section 4 (more specifically, to aninterpolation judging unit 41 to be described later).

The voice decompression section 3 may have a communication controlapparatus instead of or in combination with the storage medium drive. Inthis case, the voice decompression section 3 receives the compressiondata along with the external data from an external via a communicationline, decompresses the received compression data, and supplies the PCMsignal representative of decompression data and the received externaldata to the frequency interpolating section 4.

As shown in FIG. 7, the frequency interpolating section 4 is constitutedof an interpolation judging unit 41, an analyzer 42, an interpolatingunit 43, an interpolation band adding unit 44 and a synthesizer 45.

The interpolation judging unit 41 is made of, for example, ademultiplexer and the like. Upon reception of the PCM signal andcorresponding external data from the voice decompression section 3, theinterpolation judging unit 41 judges whether the external data indicatesthat the PCM signal contains high frequency components or not. If it isjudged that the PCM signal contains high frequency components, the PCMsignal supplied from the voice decompression section 3 is supplied tothe analyzer 42.

If the interpolation judging unit 41 judges that the external dataacquired from the voice decompression section 3 indicates that the PCMsignal does not contain high frequency components, the PCM signalsupplied from the voice decompression section 3 is output as a signaloutput from the frequency interpolating section 4.

The analyzer of the frequency interpolating section 4 shown in FIG. 7has substantially the same structure as that of the analyzer shown inFIG. 2 and performs substantially the same process as that of theanalyzer shown in FIG. 2. Therefore, the analyzer of the frequencyinterpolating section 4 shown in FIG. 7 generates (n+1) signalsrepresentative of the spectrum distributions of (n+1) bands each havingthe same band width as that obtained by dividing the spectrumdistribution of the supplied decompression data by (n+1), and suppliesthem to the interpolating unit of the frequency interpolating section 4.

The synthesizer of the frequency interpolating section 4 shown in FIG. 7has substantially the same structure as that of the synthesize shown inFIG. 5 and performs substantially the same process as that of thesynthesizer shown in FIG. 5. Therefore, the synthesizer sequentiallyoutputs the PCM signal having a spectrum distribution corresponding tothe spectrum distribution after interpolation, at substantially the sameperiod as that of the PCM signal supplied to the analyzer of thefrequency interpolating section 4.

Of the spectrum after interpolation, the spectrum of the interpolationband added by the interpolation band adding unit of the frequencyinterpolating section 4 has a spectrum distribution corresponding to thespectrum distribution of the reference band contained in the combinationof the reference band with the highest spectrum distribution correlationcoefficient and the comparison band.

(Third Embodiment)

FIG. 8 is a diagram showing the structure of a frequency interpolatingdevice according to a third embodiment of the invention.

As shown, the frequency interpolating device has substantially the samestructure as that of the frequency interpolating device of the secondembodiment shown in FIG. 6, excepting that an envelope detecting unit 5is used in place of the high frequency component detecting unit 1 andthat the frequency interpolating section 4 does not have theinterpolation judging unit 41. Similar to the frequency interpolatingsection 4 shown in FIG. 6, the frequency interpolating section 4 shownin FIG. 8 has an analyzer, an interpolating unit, an interpolation bandadding unit and a synthesizer. The operation of each component of thefrequency interpolating device of this embodiment is different from thatof the frequency interpolating device shown in FIG. 6.

The envelope detecting section 5 has, for example, an analyzer, aparallel-serial converter and a low pass filter (LPF), the analyzerhaving substantially the same structure as that of the analyzer 41 ofthe frequency interpolating section 4.

The analyzer of the envelope detecting section 5 receives a PCM signalto be compressed, generates a predetermined number of signalsrepresentative of the spectrum distribution of the PCM signal, andsupplies the generated signals to the parallel-serial converter of theenvelope detecting section 5. The PCM signal to be compressed is alsosupplied to the voice compression section 2.

Upon reception of the signals representative of the spectrumdistribution of the PCM signal to be compressed from the analyzer of theenvelope detecting section 5, the parallel-serial converter of theenvelope detecting section 5 sequentially supplies these signals to LPFof the envelope detecting section 5 in the order of lower frequency band(or in the order of higher frequency band).

Upon sequential reception of the signals representative of the spectrumdistribution of the PCM signal to be compressed from the parallel-serialconverter of the envelope detecting section 5, LPF of the envelopedetecting section 5 cuts the frequency components of the signals at thecut-off frequency or higher and supplies the other frequency components(lower frequency components) to the voice compression section 2. The lowfrequency components supplied from LPF of the envelope detecting section5 to the voice compression section 2 correspond to an envelope signal ofthe spectrum distribution of the PCM signal to be compressed.

Instead of generating the external data depending upon whether thedetection signal is supplied from the detector unit 12 shown in FIG. 6,the voice compression section 2 shown in FIG. 8 uses as the externaldata the signal representative of the low frequency components suppliedfrom the envelope detecting section 5 (an envelope signal of thespectrum distribution of the PCM signal to be compressed).

The voice compression section 2 stores the compression data and thecorresponding external data representative of the envelope of thespectrum distribution of the PCM signal before compression in anexternal storage medium set in a storage medium drive. Alternatively,the compression data and external data are transferred to an externalvia a communication line.

The voice decompression section 3 shown in FIG. 8 acquires thecompression data of the PCM signal compressed by MP3, AAC or anothermethod and the corresponding external data from the external storagemedium or from the external via the communication line. Similar to thevoice decompression section 3 shown in FIG. 6, the voice decompressionsection 3 shown in FIG. 8 decompresses the acquired compression data byMP3, AAC or another method, and supplies a PCM signal representative ofdecompression data to the analyzer of the frequency interpolatingsection 4. The acquired external data is supplied to the interpolationband adding unit of the frequency interpolating section 4.

The analyzer of the frequency interpolating section 4 shown in FIG. 8has substantially the same structure as that of the analyzer shown inFIG. 2 and performs substantially the same process as that of theanalyzer shown in FIG. 2. Therefore, the analyzer of the frequencyinterpolating section 4 shown in FIG. 8 generates (n+1) signalsrepresentative of the spectrum distributions of (n+1) bands each havingthe same band width as that obtained by dividing the spectrumdistribution of the supplied decompression data by (n+1), and suppliesthem to the interpolating unit of the frequency interpolating section 4.

The interpolating unit of the frequency interpolating section 4 shown inFIG. 8 has substantially the same structure as that of the interpolatingunit 43 shown in FIG. 7 and performs substantially the same process asthat of the interpolating unit 43 shown in FIG. 7 to determine thereference band and supply the information of the determined referenceband to the interpolation band adding unit of the frequencyinterpolating section 4.

Similar to the interpolation band adding unit 44 of the frequencyinterpolating section 4 shown in FIG. 7, the interpolation band addingunit of the frequency interpolating section 4 shown in FIG. 8 isconstituted of a DSP, a CPU and the like. Upon reception of the (n+1)signals representative of the spectrum distributions of the (n+1) bandsfrom the analyzer of the frequency interpolating section 4 and theexternal data from the voice decompression section 3, the interpolationband adding unit of the frequency interpolating section 4 shown in FIG.8 performs substantially the same process as that of the interpolationband adding unit 44 shown in FIG. 7 to supply the signal representativeof the spectrum distribution after interpolation to the synthesizer ofthe frequency interpolating section 4.

In this case, instead of performing regression calculations byidentifying the function of the envelope of the spectrum distribution ofeach band in accordance with the signal supplied from the analyzer ofthe frequency interpolating section 4, the interpolation band addingunit of the frequency interpolating section 4 shown in FIG. 8 calculatesan estimate value of a mean square value of the spectrum componentscontained in the interpolation band in accordance with the function ofthe envelope represented by the supplied external data.

The synthesizer of the frequency interpolating section 4 shown in FIG. 8has substantially the same structure as that of the synthesize shown inFIG. 5 and performs substantially the same process as that of thesynthesizer shown in FIG. 5. Therefore, the synthesizer sequentiallyoutputs the PCM signal having a spectrum distribution corresponding tothe spectrum distribution after interpolation at substantially the sameperiod as that of the PCM signal supplied to the analyzer of thefrequency interpolating section 4.

Of the spectrum after interpolation, the spectrum of the interpolationband added by the interpolation band adding unit of the frequencyinterpolating section 4 has a spectrum distribution corresponding to thespectrum distribution with the highest spectrum distribution correlationcoefficient of the reference band contained in the combination of thereference band and comparison band.

The structure of the frequency interpolating device is not limited onlyto that described above.

For example, at least some functions of the analyzer, parallel-serialconverter and LPF of the envelope detecting section 5 may be performedby DSP or CPU, or the whole function of the envelope detecting section 5may be performed by DSP and CPU. The analyzer of the envelope detectionsection 5 may be realized by a fast Fourier transform (FFT) devicehaving a well-known structure.

Instead of generating an envelope signal of the spectrum distribution ofthe PCM signal to be compressed, the envelope detecting section 5 maygenerate a signal representative of a band width occupied by the PCMsignal to be compressed. In this case, the voice compression section 2may use as the external data the data representative of the band widthoccupied by the spectrum distribution of the PCM signal beforecompression. For example, the data representative of the occupied bandwidth is constituted of the lowest frequency of the spectrum componentsof the PCM signal and the data representative of the band width occupiedby the PCM signal. If the lowest frequency of the spectrum components ofthe PCM signal is already known (e.g., 0 Hz), it is sufficient if thedata representative of the occupied band width is constituted of onlythe data representative of the band width occupied by the PCM signal.

If the external data represents the band width occupied by the spectrumdistribution of the PCM signal before compression, similar to theinterpolation band adding unit 44 of the second embodiment, theinterpolation band adding unit of the frequency interpolating section 4calculates the estimate value of a mean square value of the spectrumcomponents essentially contained in the interpolation band, byperforming regression calculations after identifying the function of theenvelope of the spectrum distribution of each band in accordance withthe signals supplied from the analyzer of the frequency interpolatingsection 4. In this case, assuming that of the interpolation bands, theband out of the occupied band width represented by the external data hasessentially no spectrum components, the estimate value of a mean squarevalue of the spectrum components essentially contained in theinterpolation band is calculated.

The frequency interpolating device of the second and third embodimentsof the invention described above can effectively achieve the secondobject of the invention.

INDUSTRIAL APPLICABILITY

As described so far, according to the invention, a frequencyinterpolating device and method can be realized which can recover asignal approximate to an original signal from a modulation wave of asignal having a limited band of the original signal, and moreparticularly can recover an audio signal with a high quality.

According to the invention, a frequency interpolating device and methodcan be realized which can properly recover a signal approximate to anoriginal signal from either an original signal with the spectrumcomponents in some bands being suppressed or a signal representative ofan original signal containing no spectrum components in the bands orfrom a signal combining these two signals.

What is claimed is:
 1. A frequency interpolating device for receiving aninput signal of an original signal whose frequency components in aparticular frequency band are suppressed, approximately recoveringsuppressed frequency components and reproducing a signal approximate tothe original signal, characterized in that short period spectra areobtained from a frequency band having frequency components notsuppressed; a short period spectrum in the suppressed frequency band areestimated by paying attention to repetition of a spectrum pattern at apredetermined frequency interval; and in accordance with thisestimation, a signal containing the frequency components in thesuppressed frequency band is synthesized and added to the input signal.2. The frequency interpolating device according to claim 1, wherein therepetition of the spectrum pattern is judged by calculating acorrelation coefficient between a spectrum pattern in a first frequencyband having a predetermined band width and frequency components notsuppressed near the suppressed frequency band and a spectrum pattern ina second frequency band adjacent to the first frequency band having thepredetermined band width, and the suppressed frequency components aresynthesized by coupling a unit of repetitive spectrum patterns.
 3. Thefrequency interpolating device according to claim 2, wherein if thespectrum pattern in the first frequency band and the spectrum pattern inthe second frequency band have a high correlation coefficient, thespectrum pattern is extended to the suppressed band to synthesize thefrequency components of the suppressed band.
 4. The frequencyinterpolating device according to claim 3, wherein the intensities ofthe frequency components to be synthesized are determined from aspectrum envelope of the suppressed frequency band estimated from aspectrum envelope of the frequency band not suppressed.
 5. The frequencyinterpolating device according to claim 1, wherein the particularfrequency band is a high frequency band.
 6. The frequency interpolatingdevice according to claim 5, wherein an upper limit frequency of thefirst or second frequency band is a lower limit frequency of asuppressed high frequency band.
 7. The frequency interpolating devicefor processing an input signal whose frequency components in aparticular frequency band are suppressed and reproducing a signal havingthe suppressed frequency band components approximately recovered, saiddevice comprising: spectrum generating means for generating short periodspectra of the input signal; spectrum pattern deriving means forderiving correlation spectrum patterns having a correlation betweenshort period spectrum patterns in adjacent frequency bands having thesame band width; spectrum envelope deriving means for deriving spectrumenvelope information in the band whose frequency components are notsuppressed; means responsive to both said spectrum pattern derivingmeans and said spectrum envelope deriving means for synthesizing asignal having frequency components in the suppressed frequency band; andmeans for adding the synthesized signal having frequency components inthe suppressed frequency band to the input signal.
 8. The frequencyinterpolating device according to claim 7, wherein the signal havingfrequency components in the suppressed frequency band has the derivedcorrelation spectrum pattern and an intensity determined by the spectrumenvelope information.
 9. The frequency interpolating device according toany one of claims 1 to 8, wherein the input signal is a PCM signalobtained by sampling and quantizing an analog audio signal.
 10. Afrequency interpolating method of receiving an input signal of anoriginal signal whose frequency components in a particular frequencyband are suppressed, approximately recovering suppressed frequencycomponents and reproducing a signal approximate to the original signal,characterized in that short period spectra are obtained from a frequencyband having frequency components not suppressed; a short period spectrumin the suppressed frequency band is estimated by paying attention torepetition of a spectrum pattern at a predetermined frequency interval;and in accordance with this estimation, a signal containing thefrequency components in the suppressed frequency band is synthesized andadded to the input signal.
 11. The frequency interpolating methodaccording to claim 10, wherein the repetition of the short periodspectrum pattern is judged in accordance with a correlation coefficientbetween a spectrum pattern in a first frequency band having apredetermined band width and frequency components not suppressed nearthe suppressed frequency band and a spectrum pattern in a secondfrequency band adjacent to the first frequency band having thepredetermined band width.
 12. A frequency interpolating method ofprocessing an input signal whose frequency components in a particularfrequency band was suppressed and reproducing a signal having thesuppressed frequency band components approximately recovered, saidmethod comprising the steps of: generating short period spectra of theinput signal; deriving correlation spectrum patterns having acorrelation between adjacent short period spectrum patterns in frequencybands having a same band width; deriving spectrum envelope informationin the frequency band not suppressed; a step of responding to saidspectrum pattern deriving step and said spectrum envelope driving stepand synthesizing a signal having frequency components in the suppressedfrequency band; and a step of adding the synthesized signal havingfrequency components in the suppressed frequency band to the inputsignal.
 13. A frequency interpolating system for receiving an inputsignal of an original signal whose frequency components in a particularfrequency band was suppressed, approximately recovering suppressedfrequency components and reproducing a signal approximate to theoriginal signal, said system comprising: means for deciding whether theparticular frequency band of the original signal contains frequencycomponents having a predetermined level or higher and generatingidentification data indicative of a presence/absence of the frequencycomponents having the predetermined level or higher; signal conversionmeans for suppressing the frequency components of the original signal inthe particular frequency band and subjecting the original signal to apredetermined signal conversion process; means for superposing theidentification data upon the converted signal to transmit theidentification data and the converted data; deciding means for receivingthe transmitted signal, checking the identification data contained inthe received signal, and deciding a presence/absence of the frequencycomponents in the particular frequency band; control means forcontrolling to input the received signal to succeeding signal processingmeans only if said deciding means judges that the particular frequencyband contains the frequency components; and signal processing meansresponsive to the received signal from said control means for performingan inverse conversion process of the predetermined signal conversionprocess and an interpolation process of approximately synthesizing andadding the frequency components in the suppressed frequency band. 14.The frequency interpolating system according to claim 13, wherein thepredetermined signal conversion process is a data compression processand the inverse conversion process to be executed by said signalprocessing means is a data decompression process.
 15. The frequencyinterpolating system according to claim 13, wherein the interpolationprocess to be executed by said signal processing means includes a shortperiod spectrum analysis process, a process of deriving short periodspectrum patterns in adjacent frequency bands having a correlation, anda process of deriving spectrum envelope information.
 16. A frequencyinterpolating device for receiving an input signal of an original signalwhose frequency components in a particular frequency band aresuppressed, approximately recovering suppressed frequency components andreproducing a signal approximate to the original signal, said devicecomprising: means for receiving a first signal obtained by subjectingthe original signal whose signal components in the particular frequencyband were suppressed to a predetermined signal conversion process and asecond signal superposed upon the first signal of identification datarepresentative of whether the particular frequency band of the originalsignal contains the frequency components having a predetermined level orhigher; deciding means for checking the identification data contained inthe received signal and judging a presence/absence of the frequencycomponents in the particular frequency band; control means forcontrolling to input the received signal to succeeding signal processingmeans only if said deciding means decide that the particular frequencyband contains the frequency components; and signal processing meansresponsive to the received signal from said control means for performingan inverse conversion process of the predetermined signal conversionprocess and an interpolation process of approximately synthesizing andadding the frequency components in the suppressed frequency band. 17.The frequency interpolating device according to claim 16, wherein thepredetermined signal conversion process is a data compression processand the inverse conversion process to be executed by said signalprocessing means is a data decompression process.
 18. The frequencyinterpolating device according to claim 16, wherein the interpolationprocess to be executed by said signal processing means includes a shortperiod spectrum analysis process, a process of deriving short periodspectrum patterns in adjacent frequency bands having a correlation, anda process of deriving spectrum envelope information.
 19. A frequencyinterpolating method of receiving an input signal of an original signalwhose frequency components in a particular frequency band aresuppressed, approximately recovering suppressed frequency components andreproducing a signal approximate to the original signal, said methodcomprising the steps of: deciding whether the particular frequency bandof the original signal contains frequency components having apredetermined level or higher and generating identification datarepresentative of a presence/absence of the frequency components havingthe predetermined level or higher; suppressing the frequency componentsof the original signal in the particular frequency band and subjectingthe original signal to a predetermined signal conversion process;superposing the identification data upon the converted signal andtransmitting the identification data and the converted data; receivingthe transmitted signal, checking the identification data contained inthe received signal, and deciding a presence/absence of the frequencycomponents in the particular frequency band; controlling to input thereceived signal to a succeeding signal processing step only if saidjudging step deciding decides that the particular frequency bandcontains the frequency components; and responsive to the received signalfrom said control step, performing an inverse conversion process of thepredetermined signal conversion process and an interpolation process ofapproximately synthesizing and adding the frequency components in thesuppressed frequency band.
 20. A frequency interpolating method ofreceiving an input signal of an original signal whose frequencycomponents in a particular frequency band are suppressed, approximatelyrecovering suppressed frequency components and reproducing a signalapproximate to the original signal, said method comprising the steps of:receiving a first signal obtained by subjecting the original signalwhose signal components in the particular frequency band were suppressedto a predetermined signal conversion process and a second signalsuperposed upon the first signal of identification data representativeof whether the particular frequency band of the original signal containsthe frequency components having a predetermined level or higher;checking the identification data contained in the received signal anddeciding a presence/absence of the frequency components in theparticular frequency band; controlling to supplying the received signalto a succeeding signal processing step only if said deciding stepdecides that the particular frequency band contains the frequencycomponents; and responsive to the received signal from said controllingstep, performing an inverse conversion process of the predeterminedsignal conversion process and an interpolation process of approximatelysynthesizing and adding the frequency components in the suppressedfrequency band.